In recent years, image display devices such as projectors and liquid crystal display devices in which laser sources are used as light sources have been researched and developed. A laser source as an approximate ideal point light source may efficiently condense light in a narrow region. Therefore, the laser source contributes to a reduction in size of an optical system incorporated in an image display device. With the laser source, the image display device is downsized.
If linearly polarized laser light is used as backlight of a liquid crystal display device, the liquid crystal display device may use light very efficiently. Therefore, the liquid crystal display device may achieve low power consumption.
The liquid crystal display device includes a liquid crystal panel for displaying images. The liquid crystal panel spatially modulates a deflecting direction of linearly polarized light to display images. Therefore, the linearly polarized light is incident on the liquid crystal panel.
A lamp or an LED, which irradiates randomly polarized light, may be used as a light source of the liquid crystal display device. In this case, the liquid crystal display device includes a polarization filter configured to convert the randomly polarized light into linearly polarized light. However, the polarization filter absorbs or reflects a part of light from the light source.
If the light source configured to emit linearly polarized laser light is used as a light source of a liquid crystal display device as described above, the liquid crystal display device does not require the polarization filter. Unless the liquid crystal display device includes the polarization filter, the polarization filter causes little optical loss. Therefore, the liquid crystal display device may efficiently use light to achieve low power consumption.
Typically, the image display device creates images with the three primary colors. Therefore, the image display device includes a red laser source configured to emit light of a red hue, a green laser source configured to emit light of a green hue, and a blue laser source configured to emit light of a blue hue. Typically, semiconductor lasers are used as the red and blue laser sources. Red and blue laser beams are generated at high power by the semiconductor lasers.
Unlike the red and blue laser sources, the green laser source has difficulties in materials for generating laser light. In the current situation, there is no material composition which generates green laser light at high power and is practically applicable to the semiconductor laser.
A wavelength conversion device is proposed to generate high power green laser light by means of a combination of a solid-state laser device and a wavelength conversion device, instead of the semiconductor laser emitting green laser light. The solid-state laser device generates fundamental light. The wavelength conversion device converts wavelength from the fundamental light into harmonic light to generate high power green laser light (c.f. Patent Documents 1 and 2). Development for mass production is in progress for wavelength conversion devices. It should be noted that the solid-state laser device means a structure for generating laser light using a laser medium. A solid-state laser device of a semiconductor laser excitation system excited by a semiconductor laser is exemplified as the solid-state laser device.
Patent Document 1: JP 4,144,642 B
Patent Document 2: WO 2009/047888